This application claims the priority of Japanese Patent Applications No. H9-231282 filed on Aug. 27, 1997, No. H9-364921 filed on Dec. 18, 1997 and No. H9-364922 filed on Dec. 18, 1997, which are incorporated herein by reference.
The present invention relates to spark plugs to be used for internal combustion engines and, more particularly, to a spark plug into which a resistor for prevention of occurrence of radio frequency noise is incorporated.
As this type of spark plug, there has conventionally been known one having a structure that a terminal is inserted and fixed into a through hole formed along the axial direction of an insulator from one end side of the through hole while a center electrode is similarly inserted and fixed thereinto from the other end side of the through hole, where a resistor is placed between the terminal and the center electrode within the through hole. Between the resistor and the terminal or between the resistor and the center electrode, generally, an electrically conductive glass seal portion for joining together the two members is placed. This spark plug is manufactured, for example, by the following process.
That is, into the through hole of the insulator, after the center electrode is inserted, conductive glass powder is filled, then material powder of the resistor composition is filled, and further conductive glass powder is filled again. As a result, in the through hole, a conductive glass powder layer, a resistor composition powder layer and another conductive glass powder layer are formed in this order from the center electrode side. Then, in this state, the insulator is heated above the glass softening point, and further a terminal is press fitted into the hole from the side opposite to the center electrode, in which arrangement the stacked layers are pressed axially so as to be compressed and sintered, thus forming a conductive glass seal portion, a resistor and another conductive glass seal portion, respectively.
In the above spark plug manufacturing method, it has been practiced that after the conductive glass powder layer, the resistor composition powder layer and another conductive glass powder layer are formed one after another and heated with the center electrode downside, the individual layers are compressed in one axial direction from the side opposite to the center electrode, by which the glass seal portions and the resistor are formed. In this case, there are some cases where enough pressing force does not act on the conductive glass powder layer located at the lowest side because of friction between the upper filler material and the through hole wall surface, so that the compression or fluidization after the glass melting and moreover the sintering of the conductive glass powder layer do not proceed enough. If the spark plug is used for a long time in such a state, it may occur that carbon in the conductive glass seal portion burns out or the metal component oxidizes so that the conducting state between the resistor and the center electrode becomes incomplete, causing the conduction resistance to increase, which may obstruct the normal ignition.
Also, when the outside dimensions of the insulator are specified by specifications of the spark plug or the like, increase in the length of the resistor is limited. In this case, one available method would be that the place of the protruding-portion receiving surface is moved toward the front end of the center electrode, so that the axial length of the second portion of the through hole is extended, by which the length of the resistor is increased by the portion. However, this method has a disadvantage that the insulator is thinned in wall thickness at the position of the protruding-portion receiving surface so that the strength of this portion is likely to lack. In this case, the crossing portion between the protruding-portion receiving surface and the second portion may serve as a kind of notch, which often causes problems in terms of strength.
Next, there is a further problem other than the above. That is, in the conventional spark plug, as shown in FIG. 16, the taper angle of a center electrode receiving portion 104 formed in a through hole 103 of an insulator 102 (where the taper angle is an angle as viewed from a reference surface that crosses the center axis line of the through hole 103) is set to 20.degree.-40.degree.. Then, the present inventors have found out that the conventional spark plug (in which the taper angle of the center electrode receiving portion 104 is 20.degree.-40.degree.) has the following disadvantage. That is, because a clearance (seal portion) 109 between an electrode-fixing protruding portion 105 and a through hole inner wall 107 of a proximity 106 of a center electrode receiving portion is narrow and deep, a conductive glass powder layer 108 in the glass seal is not fluidized enough, so that the conductive glass seal portion 109 is liable to become nonuniform in compactness. In particular, the conductive glass powder layer 108 of the proximity 106 of the center-electrode receiving portion is liable to lower in vitrifiability, which may cause a problem of deterioration in thermal conductivity. Further, poor vitrifiability of the conductive glass seal portion 109 would lead to deterioration in heat performance (anti-preignition performance) of the spark plug, while occurrence of nonuniformity in compactness of the conductive glass seal portion 109 would lead to variations in the heat performance from product to product, as further disadvantages.